Field of the Invention
The present invention relates to an oscillator circuit having a charge storage device, an upward integration current source and a downward integration current source which are each connected to the charge storage device for charging and discharging, respectively, and a comparator which is connected to the charge storage device for driving the upward and downward integration current sources as a function of a lower comparator threshold and an upper comparator threshold.
Integrated oscillator circuits such as these, which operate on the basis of the relaxation principle, are described, for example, in the article “A 1.2-μm CMOS Current-Controlled Oscillator”, Michael P. Flynn, IEEE Journal of Solid-State Circuits, Vol. 27, No. 7, July 1992, pages 982 et seq. In this case, the charge storage device is a capacitor across which a triangular waveform voltage is produced by charging and discharging the capacitor.
An oscillator circuit is also described in the article “A novel low voltage low power oscillator as a capacitive sensor interface for portable applications”, Guiseppe Ferri, Sensors and Actuators 76 (1999) pages 437-441.
The principle of a univibrator on which these oscillators are based is described in Tietze, Schenk: Halbleiterschaltungstechnik [Semiconductor Circuit Technology], 10th Edition, Springer-verlag, page 189, FIG. 8.54.
The described oscillators have the disadvantage that they require a large area on the chip surface since high-value resistors are required to produce the oscillator circuits. Furthermore, the described oscillators have a relatively high current draw as a result of the restriction of the chip surface area for the required resistors and the current draw for producing reference voltages, reference currents and comparator currents. The relatively high current draw of the described oscillator circuits is due to the fact that there are a large number of other current paths in addition to the current path that is required for charging the integration capacitance.
In order to provide a standby mode for an integrated circuit, in which the circuit switches back to a normal mode after a time that can be predetermined, it is necessary to keep an internal oscillator operating all the time. This internal oscillator is subject to requirements for a low current draw and a low operating voltage, and for it to be independent of operating voltage fluctuations, temperature fluctuations and technical scatters. A further goal is to produce an oscillator that occupies little chip surface area and can be produced cost-effectively.